1. Field of the Invention
The present invention relates to protection circuits for power transistors, and more particularly, to protection circuits used to deactivate a transistor in the event of a short-circuit in the output of the transistor that has an inductive component.
2. Description of the Related Art
Large power transistors are often used for switching loads. For example, a power transistor may be used for connecting a load to ground potential in order to draw current through the load. Such uses of power transistors are common in automotive electrical systems.
One, two, or several power transistors are typically integrated into a single semiconductor chip. One of the inputs on such a chip receives a digital switching signal that is used for activating and deactivating the power transistor, i.e., switching it on and off. The main output of such a chip is typically the collector of the power transistor. This output is normally connected to the load.
The emitter of the power transistor is coupled to ground internally within the chip, and the base is normally coupled to some type of main activation and deactivation circuitry that is at least partially controlled by the switching signal to activate and deactivate the power transistor.
Although theoretically the power transistor will function properly if connected to the proper load, there is always the practical danger that the output of the chip, i.e., the collector of the power transistor, will accidentally be short-circuited by connecting it directly to a power source. Various protection circuits have been devised in the prior art to guard against such short-circuiting. Such protection circuits typically include some type of deactivation circuit that deactivates the power transistor, i.e., turns it off, in the event of a short-circuit.
It has been found, however, that typical short-circuit protection circuits will not protect a power transistor from a short-circuit having an inductive component caused by five to ten or more feet of wire. Such short-circuits are common in automotive applications where cables having lengths of five to ten feet are frequently employed.
Thus, there is a need for a protection circuit that will deactivate a power transistor when its output is shorted to a power source with five to ten or more feet of wire.